The use of cold-cathode fluorescent lamps for general applications is well known in the art. U.S. Pat. No. 6,135,620, incorporated by reference herein, is one example showing the use of such lamps in EXIT signs, traffic signals, light bulbs and for general lighting applications.
Cold-cathode fluorescent lamps are desirable since they are more efficient and offer savings in both energy consumption and operating expenses. Such cold-cathode lamps operate at a very low current (although the voltage is quite high) and they have a longer life when compared with standard fluorescent lamps. Cold-cathode fluorescent lamps are low power consumption lamps with long life expectancy and low maintenance requirements. These lamps are often used to provide background illumination in various lighting applications. Cold-cathode fluorescent lamps require a substantial AC voltage typically several hundred volts, in order to achieve optimal light output. Although the frequency at which a cold-cathode lamp is driven varies, typically they are operated at a drive current frequency on the order of about 10–50 KHz, and have a life expectancy of 15,000–25,000 hours.
One difficulty with these types of lamps, as addressed by the above patent, is in making it easy to install and replace them. Generally cold-cathode fluorescent lamps are stand-alone tubes having a variety of plugs and wires extending therefrom for insertion into a dedicated socket. Due to the fragility of the lamp itself, extreme care must be taken during such installation as well as in shipping, handling and storage. Additionally, as indicated above, voltages required for cold-cathode fluorescent lamp operation are generally high (upwards of 1500 VAC) and thus inserting such a lamp when the fixture is still powered creates the potential for electrical shock.
Additionally, the lamp assembly using an edge-lit panel for emergency light signs such as a lighted exit sign is well known in the art. Typically, the panel includes a front side and back side having two layers of paint, a colored layer for legend and a white layer for the background. The light coming through illuminates both the legend and the background while traveling through multiple reflections and refractions on panel surfaces.
Traditionally, a prior art edge-lit panel 50 of an exit or any other sign as shown in FIG. 5 consists of a glass or plastic (acrylic) panel having a front side 50a and backside 50b. The panel 50 typically has two layers of paint (screening), a layer 52 for the legend text (usually red or green color) and a white layer 54 for the background. The layers 52 and 54 are screened on the backside 50b of the panel (the legend is in a mirror). As an option the legend may have an engraved contour 59 which is typical .1″ depth and width on the backside of the acrylic panel 50 to accentuate the contrast between the legend and background. The light 56 as shown by vertical lines falls from the top edge of the panel 50 illuminates both the legend 52 and background 54 while traveling through multiple reflections 57 and refractions 58 on the surfaces of the panel 50.
This method has several disadvantages. First, poor uniformity of the background illumination (shadow around the legend letters), poor contrast ratio between the legend 52 and background 54, and a certain colored light migration (pink shadow) from the legend 52 to the surrounding background 54, due to the multiple light reflections 57. In order to meet the UL924 visibility requirements for exit signs, the edge-lit panel 50 must be exposed to intense light sources (e.g.: two 8-Watt fluorescent lamps) which consume electrical power and dissipate heat.
In U.S. Pat. No. 4,435,743, there is shown an edge-lit sign or panel that has a translucent light reflecting layer formed of white paint applied to the backside. Further, two opaque layers are applied to the front side of the device. In this method, the legend is not internally illuminated, which makes the panel look black and white, i.e., the legend remains black in complete darkness. Therefore, this design would not meet the requirements for a colored legend in both stand-by (AC) and emergency mode.
Therefore, it is thus desirable to make an edge-lit panel to overcome these disadvantages to provide an improved uniform brightness on the panel surface.